Incineration is a technology which is becoming essential for the removal of household or municipal waste. The incineration of household waste is accompanied by the formation of a flue gas generally comprising hydrogen chloride. It is essential to remove the hydrogen chloride from the flue gas before discharging the latter to the atmosphere.
During the combustion of coal, for example in order to produce electricity, flue gases are emitted comprising sulphur oxides as acidic impurities.
A known process for purifying a flue gas comprising acidic compounds consists in treating the flue gas with sodium bicarbonate, so as to neutralize hydrogen chloride or sulphur oxides and to form sodium chloride or sodium sulphates.
More particularly, a process has been provided in which sodium bicarbonate is injected in the powder form into the flue gas exiting from the incinerator and the flue gas thus treated is subsequently sent to a filter (Solvay & Cie, brochure TR 895/5c-B-1-1290). In this known process, the flue gas has a temperature of less than 260° C. at the point of injection of the sodium bicarbonate. The latter is employed in the form of a graded powder obtained by grinding, 90% by weight of which is in the form of particles with a diameter of less than 16 μm.
In practice, the sodium bicarbonate powder employed in this known process also comprises sodium carbonate.
WO 95/19835 discloses a reactive composition based on sodium bicarbonate which has a high bicarbonate content and which is provided in the form of a powder formed of particles, the particles having a specific distribution of dimensions. This composition is remarkably effective but is expensive.
In order to be able to generalize the application of purification treatments to the flue gases produced by industrial processes, it is important in many cases to reduce the cost of the reactants used.
EP 0 858 429 describes a composition comprising at least 80% of sodium bicarbonate, less than 20% by weight of sodium carbonate, from 0.2% to 0.7% by weight of ammonia, expressed as ammonium ions, and 2 to 10% by weight of water. This composition, obtained by heat treatment of crude bicarbonate from an ammonia-soda plant, however releases high amounts of ammonia (NH3) when stored, in particular in closed atmospheres. This generates rapidly ammonia concentrations above toxicity thresholds that are detrimental to the health of people handling such composition.
The invention is targeted at providing a reactive composition based on sodium bicarbonate which can be used in treatments for the purification of flue gases, presenting high amount of ammonia compounds useful in flue gas mitigation of nitrogen oxides by catalytic conversion to nitrogen, though releasing less ammonia during storage and handling than known compositions of the prior art. Moreover, the invention relates also to a process that renders possible to produce this composition under advantageous economic conditions.